The present invention relates to a method and apparatus for forming a resist pattern.
As semiconductor element integration has increased along with research and development of a VLSI (Very Large Scale Integration), there has arisen a demand for a resist micropattern formation technique with high precision. For this reason, allowable dimensional precision has become stricter. In the most advanced technique, for example, a precision of 3.sigma..ltoreq.0.1 .mu.m (where .sigma. is the standard deviation with respect to the average size of the wafer) is required in a 6" mask substrate or 5" wafer. Furthermore, in order to use the resist pattern in a mass production line, a dimensional variation in the resist pattern between the mask substrates or wafers must be smaller than 3.sigma..ltoreq.0.15 .mu.m. A highly sensitive resist pattern is required to improve the mass production effect. The sensitivity of the resist pattern must also be controlled to be suitable for an exposure apparatus (energy rays irradiating apparatus).
FIG. 1 shows a block diagram of a conventional resist pattern formation process. A resist material is applied on a starting substrate such as a mask substrate by a spin coating method. The resist film on the substrate is heated by a heating means such as an oven or a heating plate to a predetermined temperature (Tb) in accordance with the type of resist. In other words, so-called prebaking is performed. After prebaking is performed for a predetermined period of time, the substrate having the resist film thereon is vertically supported on a support and is naturally cooled to room temperature for about 20 to 30 minutes. Electromagnetic waves within a predetermined wavelength range such as ultraviolet rays; or particle beam such as electron beam having predetermined energy selectively are irradiated on the substrate for exposure. Thereafter, predetermined development and rinsing are performed to form a desired resist pattern.
However, according to the conventional resist pattern formation method described above, the sensitivity of the resist film on the substrate becomes nonuniform, so that it is very difficult to obtain a resist pattern with high precision. In this manner, since adjustment of the sensitivity of the resist pattern is difficult, the resist can be used only in limited conditions. As a result, the resist patterns cannot be formed under proper conditions.
The present inventors have made extensive studies on the dimensional precision of the resist pattern obtained by the conventional method. As a result of such studies, we found that since the substrate having the resist film is naturally cooled after prebaking while it is vertically supported on the support, a thermal hysteresis of the resist film is nonuniform, and that the dimentional variation of the resist film is caused by the nonuniform thermal hysteresis.
A temperature profile of the entire area of the resist film during natural cooling while it was vertically supported on the support was measured by an infrared radiation thermometer. The test results are shown in FIG. 2. Referring to FIG. 2, reference symbols T.sub.1, T.sub.2 and T.sub.3 denote isotherms on the surface of a substrate 1 where T.sub.1 &gt;T.sub.2 &gt;T.sub.3. As is apparent from FIG. 2, a temperature of the lower portion of the substrate 1 is lower than that of the upper portion thereof. This temperature profile changes as a function of time. Changes in temperatures at points A, B and C of the upper, central and lower portions of the substrate 1 are measured, and the results are shown in a graph of FIG. 3. In the graph shown in FIG. 3, curve A indicates temperature at point A; curve B indicates temperature at point B; and curve C indicates temperature at point C. As is apparent from the graph of FIG. 3, the upper portion of the substrate 1 takes longer time to cool than does the lower portion thereof. A maximum difference between the temperatures at points A and B was about 15.degree. C.; and a maximum difference between the temperatures at points A and C was about 30.degree. C. This temperature profile results from the fact that the substrate is vertically supported on the support and natural convection from the lower portion to the upper portion of the substrate 1 tends to occur and heat is quickly dissipated from the lower portion of the substrate as compared with the heat dissipation at the upper portion thereof.
The resist film on the substrate 1 which was subjected to natural cooling was exposed and developed to form a resist pattern. The size of the resist pattern was precisely measured. The resist pattern size was found to have a close correlation with the temperature profile or cooling curve. The size at points A, B and C were 2.0 .mu.m, 1.9 .mu.m and 1.8 .mu.m, respectively. The sensitivities at points A and B of the resist film were measured, and the results are shown in FIG. 4. Curve A' is the sensitivity at point A; and curve C' is sensitivity at point C. As is apparent from the graph of FIG. 4, the temperature profile or cooling curve during natural cooling has the close correlation with the sensitivity of the resist film. This correlation results in dimensional variation.
As described above, according to the conventional technique, cooling process of the resist film after prebaking is not properly controlled, so the sensitivity of the resist film locally varies in accordance with cooling conditions. As a result, it is difficult to form the resist pattern with high precision.